Pgmmedian User Manual(1) General Commands Manual Pgmmedian User Manual(1)
NAME
pgmmedian - apply a median filter to a PGM file
SYNOPSIS
pgmmedian
[-width=n]
[-height=n]
[-type=median_type]
[-cutoff=int]
[pnmfile]
Minimum unique abbreviation of option is acceptable. You may use dou-
ble hyphens instead of single hyphen to denote options. You may use
white space in place of the equals sign to separate an option name from
its value.
DESCRIPTION
This program is part of Netpbm(1).
pgmmedian applies a median filter to a PGM image, using either the his-
togram sort or select kth value method to determine the median.
A median filter is a convolution filter in which the value of a pixel
in the output is the median of a certain set of pixels in the neighbor-
hood of the corresponding input pixel. The effect is to eliminate lo-
cally extreme values. Such pixels typically show up as speckles.
Pixels at the edges of the image, pixels where the convolution kernel
would go off the edge of the image, are just copied. For example, if
-height is 9, the first 4 and last 4 rows of the input image are just
copied to the output.
See the -type and -cutoff options for information on how pgmmedian
chooses between the two methods.
OPTIONS
In addition to the options common to all programs based on libnetpbm
(most notably -quiet, see
Common Options ⟨index.html#commonoptions⟩ ), pgmmedian recognizes the
following command line options:
-cutoff int
This option provides the cutoff value that pgmmedian uses to de-
cide between using the histogram sort or select kth value method
to find the median.
If (maxval / ((width * height) - 1)), where maxval is the maxval
of the image and width and height are the dimensions of the
mask, is less than the cutoff value, pgmmedian uses histogram
sort. Otherwise, it uses kth value.
This option has no effect if you specify -type.
The default is 250
-width=n
Width of the median mask to apply.
Maximum allowed is the width of the input image.
Default is 3.
-height=n
Height of the median mask to apply.
Maximum allowed is the height of the input image.
Default is 3.
-type median_type
This option selects which method to use to find median regard-
less of cutoff value. Choices are histogram_sort and select.
By default, pgmmedian decides which method to use as described
under the -cutoff option.
REFERENCES
• "Collected Algorithms from ACM" Volume II, Algorithm 489 by
Robert W. Floyd
• "A Fast Two-Dimensional Median Filtering Algorithm" in "IEEE
Transactions on Acoustics, Speech, and Signal Processing" Vol.
ASSP-27, No. 1, February 1979
• "Digital Image Processing Algorithms" by Ioannis Pitas, Prentice
Hall, 1993 ISBN 0-13-145814-0
SEE ALSO
pgmnoise(1), pamaddnoise(1), pnmconvol(1), pgmmorphconv(1), pgm(1)
HISTORY
pgmmedian was added to Netpbm in Version 10.29 (August 2005). It had
been distributed by Mike Burns via his own web site before that (and
continued to be so).
AUTHOR
Copyright (C) 1996 by Mike Burns <burns@cac.psu.edu>
DOCUMENT SOURCE
This manual page was generated by the Netpbm tool 'makeman' from HTML
source. The master documentation is at
http://netpbm.sourceforge.net/doc/pgmmedian.html
netpbm documentation 20 January 2022 Pgmmedian User Manual(1)
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